This invention generally relates to methods and devices for making electrical connections to ultrasonic transducers. In particular, the invention relates to methods for making electrical connections to ultrasonic transducer elements through an acoustic backing layer.
A typical ultrasound probe consists of three basic parts: (1) a transducer package; (2) a multi-wire coaxial cable connecting the transducer to the rest of the ultrasound system; and (3) other miscellaneous mechanical hardware such as the probe housing, potting material and electrical shielding. The transducer package is typically produced by stacking layers in sequence.
In one type of known transducer stack, a flexible printed circuit board (hereinafter “flex circuit”), having a plurality of conductive traces connected in common to an exposed bus, is bonded to a metal-coated rear face of a large piezoelectric ceramic block. The bus of the flex circuit is bonded and electrically coupled to the metal-coated rear face of the piezoelectric ceramic block. In addition, a conductive foil is bonded to a metal-coated front face of the piezoelectric ceramic block to provide a ground path for the ground electrodes of the final transducer array. The conductive foil must be sufficiently thin to be acoustically transparent, that is, to allow ultrasound emitted from the front face of the piezoelectric ceramic block to pass through the foil without significant attenuation. The conductive foil extends beyond the area of the transducer array and is connected to electrical ground.
Next, a first acoustic impedance matching layer is bonded to the conductive foil. This acoustic impedance matching layer has an acoustic impedance less than that of the piezoelectric ceramic. Optionally, a second acoustic impedance matching layer having an acoustic impedance less than that of the first acoustic impedance matching layer is bonded to the front face of the first matching layer. The acoustic impedance matching layers transform the high acoustic impedance of the piezoelectric ceramic to the low acoustic impedance of the human body and water, thereby improving the coupling with the medium in which the emitted ultrasonic waves will propagate.
To fabricate a linear array of piezoelectric transducer elements, the top portion of this stack is then “diced” by sawing vertical cuts, i.e., kerfs, that divide the piezoelectric ceramic block into a multiplicity of separate side-by-side transducer elements. During dicing, the bus of the flex circuit is cut to form separate terminals and the metal-coated rear and front faces of the piezoelectric ceramic block are cut to form separate signal and ground electrodes respectively. Electrically and acoustically isolated, the individual elements can now function independently in the array. Although the conductive foil is also cut into parallel strips, these strips are connected in common to the conductive foil portion that extends beyond the transducer array, which conductive foil portion forms a bus that is connected to ground. Alternatively, the flex circuit can be formed with individual terminals instead of a bus and then bonded to the piezoelectric transducer array after dicing.
The transducer stack also comprises a mass of suitable acoustical damping material having high acoustic losses. This backing layer is coupled to the rear surface of the piezoelectric transducer elements to absorb ultrasonic waves that emerge from the back side of each element so that they will not be partially reflected and interfere with the ultrasonic waves propagating in the forward direction.
A known technique for electrically connecting the piezoelectric elements of a transducer stack to a multi-wire coaxial cable is by a flex circuit having a plurality of etched conductive traces extending from a first terminal area to a second terminal area in which the conductive traces fan out, i.e., the terminals in the first terminal area have a linear pitch greater than the linear pitch of the terminals in the second terminal area. The terminals in the first terminal areas are respectively connected to the individual wires of the coaxial cable. The terminals in the second terminal areas are respectively connected to the signal electrodes of the individual piezoelectric transducer elements.
As the system demands on element count in these devices increase, the requirements for making electrical connection to new complex transducer geometries become more demanding. In particular, the density requirements of the transducer array are challenged by the transducers needed for multi-dimensional imaging. These transducers require elements in two dimensions, instead of the one-dimensional designs required by conventional imaging apparatus. When the electrical interconnect becomes two-dimensional, however, the designer is faced with the challenge of providing an electrical interconnect for transducer elements which are no longer accessible from the sides of the array, which is a feature common to most conventional transducer designs. More specifically, in the case of an array of three or more rows of transducer elements, one or more rows are in the interior of the array with access blocked by the outermost rows of the array. In order to connect the internal elements, complicated methods have been proposed and developed. One solution, embodied in diverse transducer designs, is to make electrical connections through the acoustic backing layer of the transducer stack.
The acoustic backing layer or plate is typically made of acoustically attenuating material that dampens the acoustic energy generated by the piezoelectric transducer in the direction away from the patient being scanned. An acoustic backing layer is typically cast from epoxy mixed with acoustic absorbers and scatterers, such as small particles of tungsten or silica or air bubbles. The mixtures of these materials must be controlled to give the acoustic backing layer a desired acoustic impedance and attenuation. This acoustic attenuation, along with the acoustic impedance, affects transducer performance parameters such as bandwidth and sensitivity. Therefore, the acoustic properties of the backfill material must be tailored to optimize the acoustic stack design. Meanwhile, the backfill material must also provide both mechanical support for the diced transducer array and, in the case of a two-dimensional array, allow for electrical connectivity to each of the individual transducer elements. The addition of the latter requirement for two-dimensional arrays presents some a typical constraints on the design and manufacturability of the acoustic backing layer. Electrical connectivity must be achieved through the acoustically attenuating material in such a manner as to prevent element-to-element electrical crosstalk. Meanwhile the electrical connector must also displace a minimal volume percentage of the acoustically attenuating material in order for the overall acoustic design of the system to be maintained.
U.S. Pat. No. 5,267,221 describes an acoustically attenuating material that contains conductive elements aligned in one direction through the acoustic material to provide electrical connectivity between a diced transducer array and an electrical circuit. The block of acoustically attenuating material spanned by the electrical conductors may be either homogeneous or heterogeneous in composition. The electrical conductors embedded within the acoustic material may be wires, insulated wires, rods, flat foil, foil formed into tubes or woven fabric. This patent also discloses forming a thin metal coating on cores made of acoustic backing material. Electrical contact to the transducer array interface may be at one or multiple locations on the array face.
A second approach for obtaining a composite acoustically attenuating material is described in U.S. Pat. No. 6,043,590, which teaches an acoustic backing block comprised of a metallized flex circuit possessing conductive traces embedded within an acoustically attenuating material.
A different approach is taken in U.S. Pat. No. 6,266,857, which discloses the formation of a set of vias and indented pad seats in an acoustically attenuating backing layer, e.g., by means of laser machining. The machined substrate is then plated with an electrically conductive material. Excess electrically conductive material is removed from the substrate to leave electrically conductive material plated on the indented pad seats and the vias, thereby forming conductive pads and plated vias, the latter constituting conductive traces that penetrate the substrate in the thickness direction. In addition, vias are formed in the piezoceramic layer and plated, these plated vias being aligned with and electrically connected to those plated vias in the backing layer that are connected to ground. This arrangement allows the electrical connection of ground electrodes on the front surface and signal electrodes on the rear surface of the transducer element array to a flex circuit on the back surface of the backing layer.
There is a continuing need for two-dimensional ultrasonic transducer arrays of improved design with electrical connection through the acoustic backing layer.